Counting direct-conversion X-ray detectors or integrating indirect-conversion X-ray detectors can be used in X-ray imaging, for example in computed tomography, angiography or radiography.
X-rays or photons can be converted into electric pulses in direct-conversion X-ray detectors by way of a suitable converter material. The converter material used can for example be CdTe, CZT, CdZnTeSe, CdTeSe, CdMnTe, InP, TlBr2, HgI2, GaAs, Si or other materials. The electric pulses are evaluated by evaluation electronics, for example an integrated circuit (application specific integrated circuit, ASIC). In counting X-ray detectors, incident X-rays are measured by counting the electric pulses triggered by the absorption of X-ray photons in the converter material. The height of the electric pulse is generally proportional to the energy of the absorbed X-ray photon. This enables the extraction of spectral information from a comparison of the height of the electric pulse with a threshold value. Generally, a scattered radiation grid is embodied on the radiation incidence side of the X-ray detector. The scattered radiation grid suppresses or reduces the detection of the X-ray photons scattered in the object. This enables image artifacts to be reduced.
Known from DE 10 2014 216 756 A1 is a first X-ray projection with a first distribution of first intensity values, which is recorded by an X-ray detector with a plurality of detector elements, wherein these include a collimator and an X-ray source that interacts with the X-ray detector. Each of the first intensity values is assigned to one of the detector elements in each case. A determination of shading of the detector elements by the collimator enables the localization of foci of the first intensity values. Herein, the determination of the shading on the first distribution and the localization step is based on the previously determined shading. Furthermore, each focus is assigned to one of the detector elements in each case.
Known from DE 10 2014 201 772 A1 is a direct-conversion X-ray detector, which comprises a semiconductor used for detecting X-rays. The detector further comprises on the underside, i.e. on the side facing away from the X-rays, a pixelated anode attached to the semiconductor. The anode is divided into a plurality of subpixels. In each case, adjacently arranged subpixels are combined to form a square counting image pixel used for the purpose of detection. Arranged between the image pixels, there is in each case a row of subpixels. These subpixels are not used for detection, i.e. they are non-counting. The non-counting subpixels have an electrically conducting link to one another.
With direct-conversion X-ray detectors, high demands are placed on signal stability. Signal stability can be influenced by different parameters, such as, for example, the temperature, the voltage applied to the converter element, additional lighting and so forth. Signal stability can in particular include the reproducibility of signals or numerical values based on the electric pulses. Signal stability can be influenced by the stability of the focus, for example an X-ray source. An, in particular temporally, unstable focus can influence the shadow-casting by the scattered radiation grid, so that it, for example, changes over time. For example, shadow-casting by the grid wall can change due to an unstable focus such that shading of adjacent detector elements of the grid wall by the grid wall differs over time.